This invention relates to cross-linkable printing inks and coatings which have particular usefulness in the manufacture of transfers, plastic films and stencils.
In recent years, the use of monomers and oligimers, both separately and as admixtures, curable by ultra violet light or by electron beam radiation has given rise to a new range of materials which are mainly used in the surface coatings industry.
The great advantage of these materials is that by blending low viscosity monomers or oligimers with high viscosity oligimers, or by suitably compounding the oligimers, it has been possible to formulate a large range of compositions which are at the correct viscosity for processing by the normal processes of coating or printing without requiring solvents.
The ability to provide surface coating compositions which are 100% non-volatile has been of great importance for a number of reasons:
First, the absence of volatile solvents ensures an environmentally friendly product.
Secondly, the use of solvents represents an additional cost since they are totally lost having once performed their purpose of reducing viscosity.
Thirdly the loss of solvents in conventional compositions such as, for example printing inks, causes variability during processing which demands greater processing control and these 100% non-volatile liquid compounds remain perfectly stable during the coating or printing processes. After the coating or printing processes have been performed, these compositions can be rapidly or instantly converted by crosslinking using ultra violet light or electron beams.
It is symptomatic of these compositions that the low viscosity monomers or oligimers which are used to adjust viscosity are not removed and are therefore finally part and parcel of the final coating or ink and affect the physical characteristics of the composition.
The formulation of inks and coatings has therefore to take into account the physical properties of the compounds both as processible liquids and finally as crosslinked coatings or inks.
In general, the availability of a range of monomers having various functionalities enables the viscosity to be adjusted whilst also controlling the softness or hardness and the flexibility or strength of the final crosslinked product. In general, monofunctional monomers do not have the high density of crosslinking as multifunctional monomers and are as a consequence more soft and flexible. Similarly the oligimers which have higher molecular weight components also vary in functionality with similar consequences.
Crosslinking of the compounds occurs either by incorporating various well known photo sensitisers which respond to particular wavelengths of light when ultra violet light is used or require no catalyst when the energy for crosslinking is derived from electron beams.
Whilst it is possible to achieve very hard, tough cross-linked films from the use of these compositions, it has, in practice, been impossible to provide tough, hard films which are also free from cracking on flexing. The resulting films do not have the resilience which can be obtained by extruded plastics in which the toughness and flexibility is provided by not only the type of polymer used but by the orientation achieved by extrusion in the molten state.
It is believed that the reason for this ultimate lack of flexibility lies in the fact that the crosslinked molecules are not orientated and are of relatively low molecular weight and are, therefore, in the form of clusters rather than extended linear molecules.
The present invention is based on the discovery that the disadvantage of the lack of flexibility in otherwise strong and tough UV or electron beam cured films can be overcome by incorporating into the liquid compositions polymeric materials in the form of powders.
According to one aspect of the present invention there is provided a cross-linkable printing ink which comprises a dispersion of a powdered polymer or resin in a liquid composition comprising at least one polymerisable monomer or oligimer.
The invention also includes a method of making a plastics film or transfer which comprises mixing a powdered polymer into a curable liquid composition comprising at least one oligimer or monomer to form an ink, printing said ink on a substrate, heating the printed ink to cause the powdered polymer to dissolve in said composition and then causing said oligimer or monomer to polymerise to form a solid, flexible, cured ink film.
In its simplest form the inks of the invention may consist of one or more powdered polymers dispersed in a single liquid polymerisable monomer. The monomer will be polymerisable to form a cured ink film or coating and the toughness and flexibility of the resulting film will depend on a number of factors, including the functionality of the monomer and its interaction with other components of the ink composition. Monofunctional monomers give very soft and sometimes tacky adhesive type cured coatings in compositions with the powdered polymer. Where the functionality is more than one a cross-linked, cured structure is normally obtained. By varying the functionality, it is possible to vary the properties of the film or coating from soft, tacky films to tough, flexible films. It is convenient to prepare a blend of liquid monomers and/or oligimers and to select components which in admixture with the powdered polymer give an ink composition of the desired viscosity and yield a final, cured ink film having the desired physical properties. For example, a monomer of low functionality will tend to introduce softness and flexibility into the cured film, while a multifunctional monomer or oligimer will cause the cured film to exhibit toughness and high strength.
The selection of the monomers and oligimers follows the above stated rules but the addition of polymeric materials in powder form requires further consideration which influence both the viscosity of the liquid form and the final crosslinked coating or ink.
If, for example, the powdered polymer is soluble in the monomer or oligimer, then the viscosity will be affected with the degree of dissolution of the powder. In practice, it is found that certain powdered polymers can be incorporated and remain in a discrete form without noticeable effect on the viscosity, provided the compositions are kept at a relatively low temperature. It is most convenient to use combinations in which the viscosity can be maintained without change for some period of time at room temperature and the powdered polymer is very soluble in the monomers or oligimers at higher temperatures.
It has been found that by selecting the monomers and oligimers or mixtures thereof and equally selecting types of powdered polymers it is possible to enhance the properties of the crosslinked coatings of inks to achieve levels of toughness combined with flexibility hitherto not possible by conventional methods.
Basically, this achievement is thought to be due to the molecular structure which can be introduced by the performed powdered polymer which is not possible when monomers or oligimers crosslink conventionally as clusters, non-linearly.
It is an object of this invention to change the normal consequences of formulation of ultra violet or electron beam curing compositions by the inclusion of powdered polymers or resins such that:
1) the compositions can be converted by coating, printing, or extruding whilst maintaining their viscosity within tolerable processing limits;
2) the powdered polymers or resins can be allowed to modify the final result by being partially or completely dissolved in the monomers or oligimers by the application of heat or other non crosslinking form of energy;
3) the solubilised polymer is permanently incorporated in the crosslinked monomer, oligimer or mixtures thereof.
Whereas it is usually preferable that the compositions covered by this invention contain no solvent whatsoever and are, indeed, 100% non-volatile and are thus extremely friendly to the environment, it is, of course, possible to reduce viscosity using normal organic solvents in which case it is usual then to evaporate the solvents before the crosslinking operation takes place.
In certain cases the solvents may still remain whilst the solubilising of the polymer takes place where the type of solvent can influence the solubility of the powder polymer. The solvent can also be water in certain cases where the crosslinking monomers and oligimers are for example water-soluble.
In most usual cases where the materials are coated, printed, or extruded it is an advantage to coat, extrude, or print the compositions to give maximum thickness and to avoid the use of solvents.
In addition to the powdered polymer, the printing ink compositions of this invention may also include one or more non-reactive polymers which may be pre-dissolved in the liquid composition or in components of the liquid composition, such as in the polymerisable monomer(s) or oligimer(s). Suitable non-reactive polymers include acrylic resins.
The compositions can be pigmented or dyed by grinding pigments into the liquid monomers or polymers, or by dissolving the dyes.
The coating or film of printing ink produced in accordance with the invention are preferably cured or the cure initiated by irradiation. Preferably the cure is effected by irradiation with visible or non-visible light or electron beam radiation. In a preferred embodiment UV light, or electron beam irradiation is employed.
Where UV light is used to effect curing, normal requirements are that the coatings must be transparent to the wavelength of UV light required for crosslinking, although in the case of electron beam radiation curing the pigmentation does not effect the cure in the same way.
If the coatings are encapsulated wet so that the coating is not exposed to the air then curing can be considerably faster. Where releasable substrates and protective overlays are used in this way then the cured films can be produced as discrete plastic sheets, reels or even small designs.
In a typical production system, the coating or film formed from an ink in accordance with the invention is pre-heated in order to cause the powdered polymer to dissolve in the liquid monomer/oligimer composition. This can be achieved, e.g. by heating with hot air, or infra-red heater or by passing a supported film over a heated roller or plate. Typically the temperature required to achieve this partial or total dissolution will be in the range of 100 to 250xc2x0 C. In some cases it may be sufficient to cause the powdered polymer to be only partially dissolved in the liquid monomer/oligimer phase at this stage.
After the pre-heating step, the coating or film is then subject to curing by irradiation with UV or electron beam heating. Additional heat may be applied at this stage. In the case of commercial UV dryers, these generally emit significant quantities of heat.
For self supporting articles for example stencils, flexible printed circuits, boxes, decorated sheets for file covers and displays, the coating or film can be cast on a conveyor from which the articles are detached after curing. Such a conveyor could include a polytetrafluoroethylene support in sheet or web form or a cylinder coated with polytetrafluoroethylene to facilitate release of the cured film.
For stickers and transfers and the like, the inks of the invention may be printed or coated on to a temporary support which might be a treated (or suitably low surface tension material) heat resistant plastic or paper.
These materials may be supported on a base or conveyor of suitably heat resistant material or may be unsupported.
The base material may be heated and cured on a carousel type printer, in which the temporary support is affixed to a plate on a rotating system and the print is first moved under a suitable heat source before being next moved under an ultra-violet light source.
Suitable monomers include ethoxylated phenol monoacrylate, tripropylene glycol diacrylate, trimethylpropane triacrylate, ethoxylated trimethylolpropane triacrylate, propoxylated glycerol triacrylate, epoxy acrylate, polyether acrylate, vinyl pyrrolidone, silicone acrylate, polyester acrylate, methacrylates, acrylic acid dimer, beta carboxyethyl acrylate, iso bornyl acrylate, ocyl acrylate decyl acrylate, aliphatic acrylates, pentaerithritol acrylates, 1,6-hexanediol acrylate and polyethylene diacrylate.
Suitable oligimers include epoxy acrylates, polyester acrylates, urethane acrylates, silicone acrylates, amine functional polyether acrylate and chlorinated acrylates.
Synergists can be used to improve crosslinking speeds such as acrylated amines.
Suitable high molecular weight powdered polymers include:
Vinyl chloride/vinyl acetate copolymers of varying degrees of polymerisation and proportions, polyesters etc.
Photoinitiators include benzophenone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one and benzildimethylketaal.
The formulations of these compositions can be generalised as follows:
In the case of electron beam radiation no photoinitiator is required. Although the best results are achieved by preheating to the solubilisation point prior to crosslinking, it has been found that a very satisfactory result can be achieved without prior heating. This is more possible with high temperatures generated from the source of light and by the exothermic reaction of the crosslinking reaction.
The following Examples are given of cross-linkable printing inks in accordance with the invention.